1. Field of the Invention
This invention relates to laser systems and more particularly to laser systems which produce frequency doubled light.
2. Description of the Prior Art
Semiconductor diode lasers are of interest for a number of applications such as optical data storage, laser printing, and biochemical analysis. One example is the gallium-aluminum-arsenide (GaAlAs) diode laser which generates laser light in the near-infrared range (750-860 nm in wavelength). In optical data storage systems, the light from the laser diode is focused onto a spot on the optical disk in order to record each bit of data. The spot size is equal to approximately .lambda./(2*(N.A.)), where .lambda. is the wavelength of the light and (N.A.) is the numerical aperture of the focusing lens. In typical systems, the (N.A.) is approximately 0.5 and the resulting spot size is approximately 800 nm in diameter.
It is apparent that if the wavelength of the laser light can be cut in half, the diameter of the spot size will also be cut in half and the overall storage density on the optical disk may be quadrupled. Unfortunately, laser diodes that produce light in the blue wavelength range (430 nm in wavelength) are not available.
One technique to convert light to a higher frequency (shorter wavelength) is known as second harmonic generation (SHG). A laser beam at a first lower frequency is passed through a nonlinear crystal, such as potassium niobate (KNbO.sub.3), which produces a second harmonic laser beam (i.e., a beam at twice the frequency of the original laser beam which entered the nonlinear crystal). This SHG technique is discussed in articles by M.K. Chun, et al., Applied Physics Letters, Vol. 55, p. 218, July 17, 1989; and P. Gunter, et al., Applied Physics Letters, Vol. 35, p. 461, Sept. 15, 1979. However, since the diode laser's output power is low, techniques to improve the second harmonic generation efficiency are required in order to produce a useful and efficient laser system.
One way to increase the efficiency of the SHG scheme is to place an optical resonator or cavity around the nonlinear crystal. The light is reflected back and forth through the crystal inside the resonator in order to generate a substantial amount of the blue light. This technique was originally proposed and demonstrated by Ashkin, et al., IEEE Journal of Quantum Electronics, Vol. QE-2, p. 109, 1966. Other examples include Goldberg, et al., Applied Physic Letters, Vol. 55, p. 218, 1989; and Baer, et al., Conference on Lasers and Electro-Optics, Paper THM5, 1989. Frequency doubling of GaAlAs diode lasers using a build-up cavity containing a nonlinear crystal such as potassium niobate (KNbO.sub.3) offers the potential for the design of simple, compact laser systems. For the build up to occur, the external cavity resonance frequency must match the diode laser frequency. Otherwise, fluctuations of the power coupled to the resonator can cause substantial fluctuations in the resulting blue laser output. Some means must be used to either control the laser frequency or the resonator frequency to maintain the frequency matching so that a stable, useful laser output can result.
Some examples of frequency matching techniques are shown in Dixon, et al., Optics Letters, Vol. 14, p. 731, 1989; R.W.P Drever, et al., Applied Physics B, Vol. 31, p. 97, 1983; and W.J. Kozlovsky, et al., IEEE Journal of Quantum Electronics, Vol. 24, p. 913, 1988. Another system is shown in a co-pending U.S. Patent Application filed Oct. 15, 1990 by the same inventors as the present invention. All of these systems experience problems in maintaining the frequency matching when the laser output must be pulsed or the power level varied. Pulsing of the laser output is a requirement for certain applications such as optical data storage. Pulsing of the diode laser input current causes the frequency of the laser light to shift away from the resonance frequency of the resonator cavity. The laser becomes unlocked from the frequency of the resonator and the system no longer produces any SHG light. SHG light will only be generated when the input current and the laser frequency is restored and stabilized to its previous value. The result is that pulsing of the entire system is relatively slow. Also, the system cannot provide SHG light at more than one power level. What is needed is a laser and nonlinear cavity system which allows for high speed pulse operation and allows for variable control of the SHG light power level.